# Topological transitivity

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A property defined for a topological dynamical system , usually for a flow or a cascade (the time runs through the real numbers or the integers). It consists of the existence of a trajectory that has the whole phase space as its -limit set. (Cf. Limit set of a trajectory; an equivalent condition is the existence of a positive semi-trajectory that is everywhere dense in .) Such a trajectory (semi-trajectory) is called topologically transitive.

Closely related to topological transitivity is the property of transitivity of domains: For any non-empty open sets there is a such that . More precisely, topological transitivity implies transitivity of domains, and the converse holds (cf. [1], [2]) if is a complete separable metric space (in this case the set of topologically-transitive trajectories has the cardinality of the continuum). Hence, with the same hypotheses on , the property of topological transitivity is symmetric with respect to the time direction: If there exists a trajectory having the whole of as its -limit set, then one has transitivity of domains and topological transitivity.

Often topological transitivity is used to mean the existence of a trajectory that is everywhere dense in . (The difference between the definitions is essential when the points of this trajectory form an open set in ; otherwise it is itself an -limit or -limit, and hence the whole of is its -limit or -limit set.) The last definition is also used for more general transformation groups [3]. The definition and some of the results also carry over to the case of non-invertible mappings and semi-groups, although one is usually not concerned with these in topological dynamics.

#### References

 [1] G.D. Birkhoff, "Dynamical systems" , Amer. Math. Soc. (1927) [2] V.V. Nemytskii, V.V. Stepanov, "Qualitative theory of differential equations" , Princeton Univ. Press (1960) (Translated from Russian) [3] W.H. Gottschalk, G.A. Hedlund, "Topological dynamics" , Amer. Math. Soc. (1955)